Optical Dissolved Oxygen Midge Temp - DO Logger
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Searching for Just the Right Sensor
Eureka scientists and engineers have been watching optical-sensor technology for over a decade, hoping to find a dissolved-oxygen sensor that could deliver Eureka-like performance under trying field conditions. Finally, we found a sensor that not only passes Eureka’s lab testing, but has operated for years in over 1000 wastewater treatment facilities. And if you’ve ever been around a wastewater plant, you know that this sensor is no weakling.
Besides its reliable, field-proven design, Eureka’s new optical dissolved oxygen (ODO) sensor offers several advantages. Because the sensor consumes no oxygen, response is fast and no stirring is needed. The typical lifespan is more than five years, during which time there are no consumable parts, caps, or membranes to replace! And because the sensor is more stable than membrane-covered sensors, calibration frequency is reduced.
You can order an ODO-equipped multiprobe today. And it’s not just any multiprobe, it’s Eureka’s reliable and economical Manta. The Manta supports ODO, temperature, pH, ORP, specific conductance, turbidity, depth, and level sensors as well.
Click here to see pictures from a six week deployment in the Gulf of Mexico, and learn how the oDO was still performing in spec.
How the ODO Operates
The principle parts of the ODO are a blue light, a photodetector, and a ruthenium compound suspended in a plastic-like material called a sol-gel. When the blue light is trained on the ruthenium compound, the compound absorbs some of the energy of the light. This extra energy changes the compound’s electron structure, but this excited state is not stable. Eventually, the electron structure returns to its original state, and releases the energy it absorbed earlier. The released energy is in the form of red light. This process is called luminescence.
The intensity of the red (released) light can be predicted if the intensity of the blue (incident) light is known or controlled. But any oxygen gas molecules in the neighborhood of the sensor will permeate the sol-gel and reduce the intensity of the red light. The photodetector, tuned for red, measures the intensity of the red light. By comparing the measured intensity of the red light to its predicted intensity (which is controlled by the intensity of the blue light), the relative amount of oxygen in the sol-gel, and therefore in the air or water surrounding the sensor, can be calculated.
Optical Dissolved-Oxygen Sensor Fact and Fiction
The ODO has been somewhat oversold in the multiparameter marketplace. One manufacturer declared the ODO insensitive to fouling, and another stated that the ODO need only be calibrated annually. Yet another claims the ODO is more sensitive than membrane-covered sensors. In fact, compared to membrane-covered sensors, the ODO is more sensitive to oxygen-active foulants (algae, for instance), and less sensitive to oxygen-inert foulants (oil, for instance). Thus, periodic maintenance remains good practice. And while the ODO is more stable than membrane-covered sensors, it’s still a good idea to check calibration every month or so. Finally, the ODO is more sensitive than membrane-covered sensors in very low oxygen levels, but less sensitive at high oxygen concentrations.
This new two parameter logger is perfect for diurnal studies. Every aspect of the Eureka Midge™ is designed with efficiency in mind. You could spend thousands of dollars on large multiparameter instruments or you could save money and buy only what you need, with the Eureka Midge’s DO and temperature measuring capabilities. And with a built-in LCD display screen, there’s no need to buy or lug around a bulky field display. More details coming soon!
http://www.eurekaenvironmental.com/manta/GSA_Pricing.htm
Eureka now offers the Ammonia / Ammonium ion selective electrode sensor. This sensor is for fresh water use only, and has an accuracy of +/- 2 mg/L so may not be suitable for your application.
A very handy feature has been added to Mantas equipped with a depth sensor. For depth sensors with a 50 meter or less rating, you can obtain a barometric pressure reading to assist with dissolved oxygen calibration. Using an Amphibian or the Manta Manager desktop software, simply hold the Manta in air and click the "Get Barometric Pressure" button. The Manta is available with 10 meter, 25m, and 50m non-vented pressure transducers as well as a 10m vented sensor. 100 meter and 200m depth sensors are available.
Manta now stores calibration data. Each time you calibrate, a log of the calibration is stored to the Manta.
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